Operation of lean burn engines, e.g. diesel engines and lean burn gasoline engines, provide the user with excellent fuel economy, and have very low emissions of gas phase hydrocarbons and carbon monoxide due to their operation at high air/fuel ratios under fuel lean conditions. Diesel engines, in particular, also offer significant advantages over gasoline engines in terms of their fuel economy, durability, and their ability to generate high torque at low speed. From the standpoint of emissions, however, diesel engines present problems more severe than their spark-ignition counterparts. Emission problems relate to particulate matter (PM), nitrogen oxides (NOx), unburned hydrocarbons (HC) and carbon monoxide (CO). NOx is a term used to describe various chemical species of nitrogen oxides, including nitrogen monoxide (NO) and nitrogen dioxide (NO2), among others.
Oxidation catalysts comprising precious metals such as platinum group metals (PGM) dispersed on a refractory metal oxide support are known for use in treating the exhaust of diesel engines in order to convert both hydrocarbon and carbon monoxide gaseous pollutants by catalyzing the oxidation of these pollutants to carbon dioxide and water. Such catalysts have been generally contained in units called diesel oxidation catalysts (DOC), or more simply catalytic converters, which are placed in the exhaust flow path from a diesel powered engine to treat the exhaust before it vents to the atmosphere. Typically, the diesel oxidation catalysts are formed on ceramic or metallic substrate carriers upon which one or more catalyst coating compositions are deposited. In addition to the conversions of gaseous HC, CO and the soluble organic fraction (SOF) of particulate matter, oxidation catalysts containing platinum group metals dispersed on a refractory oxide support promote the oxidation of nitric oxide (NO) to nitric dioxide (NO2).
As is well-known in the art, catalysts used to treat the exhaust of internal combustion engines are less effective during periods of relatively low temperature operation, such as the initial cold-start period of engine operation, because the engine exhaust is not at a temperature sufficiently high for efficient catalytic conversion of noxious components in the exhaust. To this end, it is known in the art to include an adsorbent material, which may be a zeolite, as part of a catalytic treatment system in order to adsorb gaseous pollutants, usually hydrocarbons, and retain them during the initial cold-start period. As the exhaust gas temperature increases, the adsorbed hydrocarbons are driven from the adsorbent and subjected to catalytic treatment at the higher temperature. In this regard, U.S. Pat. No. 5,125,231 discloses the use of platinum group metaldoped zeolites as low temperature hydrocarbon adsorbents as well as oxidation catalysts.
As discussed hereinabove, oxidation catalysts comprising a platinum group metal (PGM) dispersed on a refractory metal oxide support are known for use in treating exhaust gas emissions from diesel engines. Platinum (Pt) remains the most effective platinum group metal for oxidizing CO and HC in a DOC, after high temperature aging under lean conditions and in the presence of fuel sulfur. Nevertheless, one of the major advantages of using palladium (Pd) based catalysts is the lower cost of Pd compared to Pt. However, Pd based DOCs typically show higher light-off temperatures for oxidation of CO and HC, especially when used with HC storage materials, potentially causing a delay in HC and or CO light-off. Pd containing DOCs may poison the activity of Pt to convert paraffins and/or oxidize NO and may also make the catalyst more susceptible to sulfur poisoning. These characteristics have typically prevented the use of Pd as an oxidation catalyst in lean burn operations especially for light duty diesel applications where engine temperatures remain below 250° C. for most driving conditions. As emissions regulations become more stringent, there is a continuing goal to develop diesel oxidation catalyst (DOC) systems that provide improved performance, for example, light-off performance.
One way of achieving higher purification performance of exhaust gas has been to control the cluster size of the precious metal to an optimal size. In particular, according to the supporting method of the precious metal of the prior art which uses a solution of the precious metal compound, the precious metal is adsorbed on the oxide support at an atomic level in which the precious metal compound is dispersed to the surface of the oxide support. However, the atoms of the precious metal move and induce grain growth in the calcination process in which the precious metal is firmly supported. It has therefore been extremely difficult to support only the precious metal of a desired cluster size on the oxide support.
WO 2010/133309 A1 relates to a palladium enriched diesel oxidation catalyst and its application as catalyst for the oxidation of CO and hydrocarbon emissions.
WO 2010/083357 A2 concerns layered diesel oxidation catalyst composites, wherein palladium is segregated from a molecular sieve and in particular from a zeolite in a catalytic material.
WO 2010/083315 A2 concerns a diesel oxidation catalyst with a layered structure for the treatment of exhaust emissions from a diesel engine as well as to a method for treating a diesel exhaust gas stream. In particular, a catalyst structure comprising three distinct layers is disclosed therein, in which the layer comprises a precious metal component such as palladium which is located between two hydrocarbon storage layers comprising a molecular sieve such as a zeolite.
WO 2010/083313 A2 relates to a diesel oxidation catalyst composite with a layer structure comprising at least two distinct layers, at least one of which contains an oxygen storage component that is present in a layer separate from the majority of the platinum group metal components such as palladium and platinum.
WO 2008/002907 A2 concerns a diesel exhaust treatment system wherein an oxygen storage component is utilized and degradation of the oxygen storage component is correlated with the degradation of the hydrocarbon conversion efficiency of a catalyst in a diesel engine system.
However, these prior art diesel oxidation catalysts still show unsatisfactory breakthrough of hydrocarbons and carbon monoxide. Furthermore, the hydrocarbon storage capacity of selected diesel oxidation catalysts of the prior art is enhanced at the expense of the catalytic activity of the catalyst.